1. Field of the Invention
The present invention-relates to an optical recording medium for recording digital data three-dimensionally thereon by a principle of holography, a method for recording information onto the optical recording medium, a method for reproducing the information on the optical recording medium, and an information recording/reproducing device.
2. Description of Related Art
In holographic storage, two-dimensional digital data is multiplex-recorded by using two light beams of a reference beam and an object beam, thereby performing three-dimensional recording. As multiplex-recording methods, there are known angle multiplexing, wavelength multiplexing, phase code multiplexing and the like.
As a conventional method for recording data at high packing density on a recording medium, a method disclosed in JP-A 2004-4434 is known.
The publication discloses a method for recording information only in one of a plurality of regions obtained by dividing a recording layer by selectively irradiating the region with an object beam and a reference beam. In this method, different information can be recorded in different regions, so that a recording capacity per medium can be increased.
By disposing, above a recording medium, a spatial beam selector having an opening only in a portion corresponding to a region from which information is desired to be reproduced, and irradiating the recording layer with a reference beam in such a state, information recorded by the method above can be reproduced only from the desired region.
In the above-mentioned method, the spatial beam selector is not used at the time of recording information, but is disposed above a medium at the time of reproduction. Consequently, it is difficult to achieve accurate alignment between the medium and the spatial beam selector. In the case where accurate alignment between the medium and the spatial beam selector cannot be obtained, there may arise various problems such that information in a neighboring region is reproduced.
In addition, there arise the following two problems.
First, a photopolymer and photorefractive material as candidates of recording material of a holographic memory are photosensitive materials and are easily exposed to natural light like a film of a camera, so that erroneous recording, data destruction and the like will occur. Consequently, when it is assumed to use the material for a removable medium, equipment for preventing exposure is essential.
Second, from an aspect of a storage system, address information indicative of a recording/reproducing position on a medium is necessary. In the case where address information using pits and wobbles as used in a conventional optical disc medium is recorded on the same recording surface, although two-dimensional information of several tens of thousands of bits is recorded with condensed light in holographic recording, only 1-bit information can be read from a pit or wobble with the same condensed light. Consequently, the proportion of an area of a region for recording address information becomes relatively large and a substantial recording region decreases.